For reasons of comfort and safety, electrically heatable seats are used in modern vehicles. Both the drivers seat and the other seats can be so arranged that they can be heated by means of special heating elements in the form of electrically conducting wires which are placed in the shape of a heating coil in each seat. Such a heating element is normally placed in the back-rest and in the cushion of each seat when manufacturing the seat. The heating element is furthermore connected to a current feeding unit which delivers current. In this manner, the heating element can be heated to a suitable temperature.
A problem of previously known heating elements is caused by the desire for each seat to have a carefully adjusted temperature on its surface, i.e. on that surface which is in contact with the person sitting in the seat. For this purpose, the temperature of the heating element can be controlled by means of a temperature detector which is arranged in the close vicinity of the heating element, and which is connected to a central control unit. Using the temperature detector and the control unit, the current temperature can be detected. The control unit also comprises current feeding circuits which, for example, can be based on transistor or relay technology, for the feeding of current to the heating element. In this way, the central control unit is arranged to feed a certain current to the heating element until a certain desired value for the temperature is reached. The setting of this desired value can, for example, be done by means of fixed resistances or by means of an adjustable potentiometer, which is adjusted by a person travelling in the vehicle.
U.S. Pat. No. 4,700,046 discloses a control device for a heating element in a vehicle seat. The heating element can be controlled so as reach a certain set temperature.
Using the above described control method, current can be delivered to the heating element until the central control unit indicates that the desired value has been reached. When this happens, feeding of the current stops. This causes the heating element to successively cool down. When the heating element has cooled so that its temperature again falls below the desired value, current feeding to the heating element will be resumed. In this way, the temperature control will continue for as long as the system is operative.
Although this previously known system normally provides a reliable heating and temperature control for a vehicle seat, it has, however, certain drawbacks. One such drawback is due to the fact that the heating element normally is assembled in the seat of the vehicle when it is manufactured, with the heating element being adjusted according to a certain "normal" seat, with a certain given design, upholstery, etc. The seat, in this manner, comprises a heating element for the purpose of heating the surface of the seat to a certain desired temperature. However, when assembling the vehicle the manufacturer might choose to equip the seat in question with, for example, a completely different upholstery, for example a considerably much thicker upholstery than that of said "normal" seat, i.e. an upholstery which differs from that for which the temperature control was originally intended. The temperature value which is detected by the temperature sensor will reach the set desired value when the heating element has reached the desired temperature, but since there is an abnormally thick upholstery on the seat, the temperature on the surface of the seat will be too low. This situation thus causes an undesired deviation of the temperature control.
In a corresponding manner, similar problems can also occur if an abnormally thin upholstery is arranged on the seat, or if the heating element is arranged at a distance from the surface of the seat which deviates from that of the "normal" seat.